How Hubble Could Spot Quantum Foam

December 7, 2009

How Hubble Could Spot Quantum Foam

The Hubble Space Telescope may be on the verge of finding the first evidence that spacetime is quantised

The idea that spacetime is quantised at the Planck scale has been
around for almost as long as physicists have attempted to reconcile
general relativity and quantum mechanics. In the 1960s, John Wheeler
coined the term quantum foam to describe the quantised structure of
the universe at distances of around 10^-35 metres or so.

One question that has occupied physicists since then is how to
detect this foamy structure. Today, Wayne Christiansen at the
University of North Carolina at Chapel Hill and a few buddies say
that various large telescopes are on the verge of being able to make
measurements that could prove the existence of quantum foam or place
important limits on its structure.

The thinking goes like this. One of the consequences of spacetime
being quantised is that it places a fundamental limit on how
accurately distances can be measured.

So imagine that you want to measure distance using a beam of
light. This light will be influenced by the structure of spacetime
and its wavefront will acquire a similarly foam-like structure. This
limits the accuracy of the distance measurements that can be made
with this light.

It also effects the way light from a point source should appear
since the wavefront has a random foamy element to it. “In
effect, spacetime foam creates a “seeing disk”,” says
Christiansen and co, and this should be visible on images of certain
distant point sources.

The trouble is that the effect is so tiny that it is only visible
in images of objects over truly cosmological distances.

That’s OK though, say Christiansen and co, because the Hubble
Space Telescope has photographed exactly the kinds of objects that
are far enough away to demonstrate the effect. These objects known as
high redshift quasars appear in an image known as the Ultra Deep
Field.

Their paper today assesses these quasars in the Ultra Deep Field
to determine to what extent they show evidence of quantum foam.

And here’s a thing: Christiansen and pals say that these quasars
are blurred in exactly the way you’d expect from quantum foam in
certain kinds of models of the universe. “The blurring is at a
level consistent with a spacetime foam model,” they say.

But
that’s not conclusive because the blurring may also have been caused
by other effects such as some other scattering medium like dust in
the intervening distance. Another possibility is that quasar may not
be a true point source and the blurring reflects the structure of the
quasar itself.

These are questions that the next round of
observations may be able to resolve. Hubble’s recent refit has given
it the ability to make better measurements. And the Very Large
Telescope Interferometer now under construction could also make
valuable contributions when it is completed.

Which means that
we could be on the verge of finding the first evidence of the
foam-like structure of spacetime. An area to watch.